A detailed kinetic mechanism of hydrocarbon oxidation and pyrolysis, coupled with a submechanism of particle inception, coagulation, surface growth, and oxidation is used to simulate the formation of particulates in a coflowing diffusion flame of ethylene. The model predicts the volume fraction of total particulates, as measured by broadband UV-visible extinction, in different flame locations and particularly in the wing regions of the flame. The model also predicts, for the first time, particle diameter in the different regions of the flame showing the formation of nanosized precursor particles in the fuel side of diffusion flames.